The present invention relates to the manufacture of semiconductor devices incorporating micro lens structures and, more particularly to the manufacture of micro lens structures where micro lens merger and collapse phenomena are minimized.
Micro lenses are optical components on semiconductors utilized as solid state imaging and/or microdisplay devices. Generally deployed in arrays, each micro lens acts as a light-gathering or light-dispersing element associated with a single-pixel or sub-pixel element in the solid-state device. One of the most important considerations in designing and manufacturing micro lenses is photosensitivity (i.e., the ability of the lens to gather or disperse light). One reason micro lens photosensitivity may be reduced is that the area of each micro lens has been reduced below an optimum value. This reduction has been necessitated by the manufacturing techniques of the prior art that cause individual lens areas to flow into one another (lens merger) or lens collapse during a hardbake reflow operation, thereby necessitating wide inter-lens bands.
Another common problem in the manufacture of semiconductor devices with micro lenses is residue left on bond pad areas (e.g., the areas between or adjacent individual micro lenses) where electrical interconnections must be made. Bond bad residue has traditionally been treated by increasing exposure energy levels or prolonging developing time during the processing of the wafers. These solutions, however, both result in decreased surface areas of the pre-hardbake micro lens regions and, consequently, in smaller finished micro lens structures having reduced optical performance.
It is possible to reduce the problem of inter-lens/bond pad residue by adding additional process steps (e.g., an additional lithographic step). However, these steps add processing cost and introduce the possibility of lower wafer yields.
U.S. Pat. No. 5,321,297 for SOLID STATE IMAGE PICKUP DEVICE HAVING LIGHT CONVERSION LENS FORMED ON A STRIP AREA; issued Jun. 14, 1994 to Tadashi Enomoto, teaches a method wherein individual micro lens structures may be individually adjusted. Variation in x-axis vs; y-axis sensitivity due to spherical aberration and other phenomena may thereby be controlled. Specifically, a strip layer is formed above a plurality of photosensitive sections, disposed in a row direction. Light-converging lenses are formed on each strip layer to obtain a desired curvature or thickness. In contradistinction, the method of the instant invention provides no individual control of the optical characteristics of the micro lenses but rather the unique descum process step allows for much tighter control of the optical parameters of all the micro lenses in the array so that both the optical characteristics and the process yield are enhanced.
U.S. Pat. No. 5,364,498 for ETCH METHOD FOR MANUFACTURING A COLOR FILTER; issued Nov. 15, 1994 to Kun-Ti Chen teaches forming a color filter layer by the deposition of a plurality of interference films on a substrate of black matrix material. CHEN does not teach the formation of micro lens structures. The method of the present invention, however, teaches an improved process for the formation of micro lens structures over a suitable color filter.
U.S. Pat. No. 5,514,888 for ON-CHIP SCREEN TYPE SOLID STATE IMAGE SENSOR AND MANUFACTURING METHOD THEREOF; issued May 7, 1996 to Yoshikazu Sano, et al. teaches a method for forming charge-coupled devices (CCDS) on a silicon substrate. A micro lens array is formed over the CCD array using conventional lithography and reflow techniques. The inventive method, on the other hand, utilizes a unique descum step which, in combination with other process steps, minimizes the formation of faulty lens structures either because of lens merger or lens collapse. Bond pad areas are also kept substantially residue free in the inventive method.
U.S. Pat. No. 5,739,548 for SOLID STATE IMAGING DEVICE HAVING A FLATTENING LAYER AND OPTICAL LENSES; issued Apr. 14, 1998 to Yoko Shigeta, et al. teaches a resin lens structure on a solid state imaging device. A remelt step at a temperature lower than the resin melting point keeps the lens structures from flowing out (i.e., merging). No descum step is taught and, consequently, the lenses formed according to these teachings sufferer from the same failure mechanisms as with other prior art methods.
Nothing in the above-described or any other known references, taken individually or in any combination, teaches or suggests the unique descum step of the present invention.
It is the object of the present invention to provide a method for forming micro lens arrays on light-sensitive or light-emitting semiconductor structures which minimizes lens merger or lens collapse during the reflow operation. A unique descum step is performed prior to a lens reflow step resulting in a low incidence of lens merger or lens collapse. A second objective of the present invention is to provide a method in which the bond pad areas are kept free of residue which results in improved reliability of the electrical interconnections.